1. Technical Field of the Invention
The present invention relates to sensorless brushless DC (BLDC) motors and, in particular, to back EMF detection with respect to such motors.
2. Description of Related Art
Sensorless Brushless DC (BLDC) motor drives are becoming widely used as small horsepower motor controls due to their high efficiency, reliability, low maintenance and low cost. An article by J. Shao, et al., entitled “A Novel Direct Back EMF Detection for Sensorless Brushless DC (BLDC) Motor Drives,” Applied Power Electronic Conference (APEC 2002), pp 33-38, the disclosure of which is hereby incorporated by reference (“the Shao Article”), proposes a direct back EMF sensing scheme in which true back EMF signals can be directly extracted for each phase without sensing the neutral point of the motor by synchronously detecting the back EMF during PWM off time. This method demonstrates better performance than the traditional methods which are based on motor neutral voltage information (see, for example, U.S. Pat. No. 4,654,566, and T. Endo, et al., “Microcomputer Controlled Brushless Motor without a Shaft Mounted Position Sensor,” IPEC-Tokyo, 1983, the disclosures of which are hereby incorporated by reference).
However, since the direct back EMF sensing scheme of the Shao Article requires minimum PWM “off” time in order to sample the back EMF signal, the duty cycle cannot reach 100%. On the other hand, in some applications, for example, high inductance motors, the long settling time of a parasitic resonant between the motor inductance and the parasitic capacitance of power devices can cause false zero crossing detection of back EMF.
This application for patent analyzes the impact of this parasitic resonant on back EMF zero crossing detection. An improved direct back EMF detection scheme which samples the motor back EMF synchronously during PWM “on” time is proposed to overcome the foregoing problems.
Generally, a brushless DC motor is driven by a three-phase inverter with what is called six-step commutation. The commutation phase sequence is of the format AB-AC-BC-BA-CA-CB. Each conducting phase is called one step. The conducting interval for each phase is 120° by electrical angle. Therefore, only two phases conduct current at any time, leaving the third phase floating. This opens a window to detect the back EMF in the floating winding.
FIG. 1 shows the typical inverter configuration and current commutation sequence for a brushless sensorless DC motor. In accordance with the prior art method of the Shao Article, the PWM signal is applied on the high side switches (T1, T3 and T5) only, and the back EMF signal is synchronously sampled only during the PWM off time. The low side switches (T2, T4 and T6) are only switched to commutate the phases of the motor. The true back EMF can be detected during off time of the PWM signal because the terminal voltage of the motor is directly proportional to the phase back EMF during this interval. Also, the back EMF information is referenced to ground, which eliminates the common mode noise, and the synchronous sampling rejects the high-frequency switching noise. The start-up performance is good since there is no signal attenuation.
Since the back EMF is detected during PWM “off” time, a minimum off time is necessarily required for the operation to occur. In low voltage applications, like automotive fuel pumps (see, J. Shao, et al., “A Novel Microcontroller-Based Sensorless Brushless (BLDC) Motor Drive for Automotive Fuel Pumps,” IEEE Transaction on Industry Applications, November/December 2003, Vol. 39, No. 6, pp 1734-1740, the disclosure of which is hereby incorporated by reference), a 100% duty cycle operation is desired to fully utilize the battery voltage.
It has also been found that in some HVAC applications the back EMF zero crossing is not truthfully detected at high speed/high duty cycle if the motor winding inductance is high.
Still further, the parasitic capacitance of power switches (for example, IGBT's) will resonate with the motor inductance in the floating phase during PWM off time.
All of the foregoing present problems with the use of the technique disclosed by the Shao Article.
Operation of the technique proposed by the Shao Article may be better understood by reference to an example. Assume a particular step where phase A and phase B are conducting current, and phase C is floating. The upper switch of phase A (i.e., T1) is controlled by the PWM signal and lower switch of phase B (i.e., T4) is on during the whole step. The terminal voltage Vc is sensed. FIG. 2A shows a circuit equivalent for this particular example.
When the upper switch (T1) of the half bridge is turned off responsive to the PWM signal (and the lower switch T4 remains on), the current freewheels through the diode D associated with the lower switch T2 of phase A. During this freewheeling period, the terminal voltage Vc is detected using a comparator circuit as Phase C BEMF when there is no current in phase C.
From the equivalent circuit shown in FIG. 2A, it is easy to see Vc=ec+Vn, where Vc is the terminal voltage of the floating phase C, ec is the phase back EMF and Vn, at the center or neutral point of the motor, is the neutral voltage of the motor.
From phase A, if the forward voltage drop of the diode is ignored, we have Eq (1):
  Vn  =      0    -          r      ⁢                          ⁢      i        -          L      ⁢                        ⅆ          i                          ⅆ          t                      -          e      a      
From phase B, if the voltage drop on MOSFET is ignored, we have Eq (2):
  Vn  =            r      ⁢                          ⁢      i        +          L      ⁢                        ⅆ          i                          ⅆ          t                      -          e      b      
Taking Eq (1) and Eq (2) together, we obtain Eq (3):
  Vn  =      -                            e          a                +                  e          b                    2      
Also from the balanced three-phase system, we know Eq (4):ea+eb+ec=0
From combining Eq (3) and Eq (4), we obtain Eq (5):
  Vn  =            e      c        2  
So, the terminal voltage Vc is provided by Eq (6):
  Vc  =                    e        c            +      Vn        =                  3        2            ⁢              e        c            
From the above equations, it can be seen that during the off time of the PWM signal, which is the current freewheeling period, the terminal voltage of the floating phase is proportional to the back EMF voltage without any superimposed switching signals. It is also very important that this terminal voltage is directly referred to the ground instead of the neutral point. So, the neutral point voltage information is not needed in order to detect the back EMF zero crossing.
A simplified equivalent circuit for the circuit presented in FIG. 2A is shown in FIG. 2B (for when phase C is floating). At the beginning of PWM off time, the voltage Vc is:
  Vc  =                    1        2            ⁢      Vdc        +                  3        2            ⁢              e        c            This is the initial condition for the resonant between L and Coes during PWM off time. Solving the L-C series resonant equation, the time for the terminal winding voltage to settle down can be estimated. Because the parasitic resonant between the motor inductance and the parasitic capacitance of power devices, it requires a longer off time for the winding terminal voltage to settle down from the transient during PWM off time. If the PWM off time is shorter than the settling time, false zero crossing of the back EMF may be detected.
FIG. 3 shows a block diagram of a circuit for back EMF detection during PWM off time. The power stage includes the drive transistors T1-T6 shown in FIG. 1. A gate drive circuit receives control signals (such as PWM on/off signals) and generates the gate drive signals for application to the power stage. The control signals are generated by a microcontroller (for example, an ST72141 microcontroller integrated circuit from STMicroelectronics). The back EMF sensing voltages are received by the microcontroller from the three phases of the motor through a set of resistors R1-R3 and are processed by an integrated zero crossing detection circuit. More specifically, the microcontroller implements the voltage detection/comparison operation (as shown in FIG. 2A) with respect to each motor phase for purposes of making back EMF detection and sensing motor position and rotation. The microcontroller further functions to generate the appropriate control signals in response thereto for purposes of actuating the motor.
Other prior art of interest include: (a) K. Rajashekara, A. Kawamura, and K. Matsuse, “Sensorless Control of AC Motor Drives,” IEEE Press, 1996; and (b) S. Ogasawara and H. Akagi, “An Approach to Position Sensorless Drive for Brushless DC Motors,” IEEE Trans. on IA, Vol. 27, No. 5, September/October 1991.